Life-history differentiation of benthic and limnetic ecotypes in a polytypic population of threespine stickleback (Gasterosteus aculeatus)

Resource-use polymorphisms are associations between heritable phenotypic variation and habitat use within populations. These polymorphisms can offer insights into the mechanisms by which adaptation can occur despite gene flow, and into the importance of these mechanisms for the formation of new species. Resource-use polymorphisms are relatively common in northern freshwater fishes, including the threespine stickleback (Gasterosteus auleatus) complex. This complex exhibits parallel trophic variation at three nested levels: (1) species differences, (2) ecotypic variation across populations and (3) polytypy within populations. Of these levels, polytypic variation has been the least well characterized in the threespine stickleback, having been detected in only two populations (Benka Lake, Alaska; Cranby Lake, British Columbia). In each lake, individuals are specialized in body shape and trophic morphology along the benthic (bottom feeding) to limnetic (plankton feeding) axis. The morphotypes in each population differ in habitat use and diet. Here we demonstrate that in Benka Lake, Alaska, females of the two ecotypes also differ in reproductive allocation, with benthic females producing fewer, larger eggs. Our results offer the first evidence of reproductive life-history differences between morphotypes of stickleback within a single population. The difference in egg size between the morphotypes may reflect plasticity paralleling that in other aspects of the phenotype. We believe, however, it is more likely that the association between morphology and egg size is favoured by disruptive selection on fry size imposed by differences in water temperature between the habitats in which the morphotypes breed. Assortative mating in Benka Lake apparently has led to more pervasive phenotype divergence than previously suspected and possibly even to incipient sympatric speciation.